NSW Police confirmed soon after the incident that a person of interest had been identified.

"At this stage the person of interest we are looking for is still at large, but we believe this to be a targeted incident and that there are no risks to the community at this time," Acting Assistant Commissioner Brett McFadden told reporters.

"There's no other way you can describe this, it's an absolutely horrific incident to unfold."

A woman in her 30s, who police understand to be the mother of the teens, was treated for shock at the scene.

A crime scene was quickly established on Hull Road, 20km northwest of the city's CBD.

Ryde detectives are being assisted by detectives from the State Crime Command’s Homicide Squad.

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